This invention relates in general to the manufacture of slip joints, such as are commonly used in vehicle drive train systems, for transmitting rotational force or torque between telescoping members. In particular, this invention relates to an improved method for applying a low friction coating to the cooperating splines of such a slip joint that results in precise conformance between the intermeshing splines of the telescoping members.
In most land vehicles in use today, a drive train system is provided for transmitting rotational power from an engine/transmission assembly to an axle assembly so as to rotatably drive one or more wheels of the vehicle. To accomplish this, a drive shaft assembly is usually connected between an output shaft of the engine/transmission assembly and an input shaft of the axle assembly. To accomplish this, a first universal joint is connected between the output shaft of the engine/transmission assembly and a first end of the drive shaft assembly, while a second universal joint is connected between a second end of the drive shaft assembly and the input shaft of the axle assembly. The universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the drive shaft assembly to the input shaft of the axle assembly, while accommodating a limited amount of angular misalignment between the rotational axes thereof.
Not only must the drive train system accommodate a limited amount of angular misalignment between the engine/transmission assembly and the axle assembly, but it must also typically accommodate a limited amount of relative axial movement. A small amount of such relative axial movement frequently occurs when the vehicle is operated. To address this, it is known to provide one or more slip joints in the drive shaft assembly of the drive train system. A typical slip joint includes first and second splined members that are respectively connected to the output shaft of the engine/transmission assembly and to the input shaft of the axle assembly. The splined members provide a rotational driving connection between the components of the drive train system, while permitting a limited amount of axial misalignment therebetween.
As is well known in the art, one or both of the splined members may be coated with a material having a relatively low coefficient of friction. The low friction coating is provided to minimize the amount of force required to effect relative movement between the two splined members. Also, the low friction coating provides a relatively tight fit between the cooperating splines of the two splined members, thus minimizing any undesirable looseness therebetween while continuing to allow free axial movement.
A number of methods are known for applying the low friction coating to the splined members of the slip joint. In one known method, the splined region of the slip joint member is first primed, then dipped into a molten bath of the low friction coating. In another known method, the splined region of the slip joint member is initially heated, then immersed in a bed containing a quantity of the low friction coating in particulate form. Alternatively, the splines can be coated electrostatically, wherein the low friction coating is applied in a particulate form. It is also known to apply the low friction coating by injection molding the low friction material between the assembled splined members.
One problem that has been experienced in connection with slip joints manufactured according to known methods is that an undesirably large gap can still exist between adjacent splines formed on the cooperating members of the slip joint even after the coating has been applied. These relatively large gaps can occur as a result of manufacturing tolerances in the formation of the individual splined members and the coating process and usually result in an undesirable amount of looseness between the splined members during operation. Looseness that occurs in the direction of rotation of the splined members, wherein one of the splined members can rotate relative to the other splined member, is referred to as backlash. Looseness that occurs in the direction transverse to the axis of rotation of the slip joint, wherein one of the splined member can extend at a cantilevered angle relative to the other splined member, is referred to as broken back. Known solutions to the problems of backlash and broken back have been found to be relatively difficult, costly, and time consuming to employ. Thus, it would be desirable to provide an improved method for applying a low friction coating to the cooperating splines of a slip joint that results in precise conformance between the intermeshing splines of the telescoping members, thus, minimizing the adverse effects of backlash and broken back.